A linear polysulfone copolymer having a recurring unit shown by formula (A) ##STR2## wherein n is an integer of 1 or more, and a recurring unit shown by formula (B) ##STR3## is already described in Japanese Patent Publication No. 21458/71 and the sulfonated product of the above-described copolymer is already described in Japanese Patent Application (OPI) No. 48222/80; the term "OPI" indicates an unexamined published patent application open to public inspection. That is, it is described in the above-described published specification that by sulfonating the above-described polysulfone copolymer by dissolved it in concentrated sulfuric acid, a hydrophilic sulfonated polysulfone wherein the recurring units shown by formula (A) are substantially wholly sulfonated but the recurring units shown by formula (B) wholly remain as a substantially un-sulfonated state.
Also, a sulfonated product of a polysulfone having a recurring unit shown by formula (C) ##STR4## is described in U.S. Pat. No. 3,709,841. Furthermore, there is described a process of producing a composite semipermeable membrane for reverse osmosis composed of a thin membrane having semipermeability laminated on a ultrafiltration membrane by coating a solution of the above-described sulfonated polysulfone on a dense layer on the surface of an anisotropic ultrafiltration membrane and evaporating off the solvent in Japanese Patent Application Nos. 99973/75 and 146379/76. Similarly, there is described a process of obtaining a composite semipermeable membrane by previously filling the fine pores of an anisotropic ultrafiltration membrane having the recurring unit shown by above-described formula (C) with an aqueous solution of lactic acid, coating the ultrafiltration membrane with a solution of a sulfonated product of a polysulfone having the recurring unit shown by above-described formula (C), and then evaporating off the solvent in Office of Water Research and Technology Department of the Interior, Report No. 2001-20.